A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Microchip fabrication involves the control of tolerances of a space or a width between features, and/or between elements of a feature such as, for example, two edges of a feature. In particular, the control of space tolerance of the smallest of such spaces permitted in the fabrication of the device or IC layer is of importance. Said smallest space and/or smallest width is commonly referred to as the critical dimension (“CD”).
In order for the devices formed by the lithographic process to function correctly and consistently, it is important to ensure that the CD of a completed device or part of a completed device is as close as possible to a desired value. It is also important to ensure that the CD value varies as little as possible between devices formed on different substrates, between devices formed on different parts of a single substrate and between different parts of a single device. The CD may vary between substrates and across substrates, however, because of variations in the processing conditions of the resist on the substrate, for example. The inherent CD variation may be compensated for by adjusting the dose applied to the substrate.
However, it has also been found that the CD may vary between features formed on the same part of the substrate. In particular there may be a difference in the CD of isolated features (namely features formed on the substrate in regions of relatively low pattern feature density) and dense features (namely features formed in regions of relatively high pattern feature density). These variations may limit the process latitude, i.e. the available depth of focus in combination with the allowed amount of residual error in the dose of exposure of irradiated target portions for a given tolerance of the CD. This problem arises because features of the patterning device having the same nominal critical dimensions may print differently depending on their pitch (i.e. the separation between adjacent features) due to pitch dependent effects. For example, the feature consisting of a line having particular line width when in isolation, i.e. having a large pitch, will print differently from the same feature having the same line width when together with other lines of the same line width in a dense arrangement on the patterning device, i.e. having a small pitch. Hence, when both dense and isolated features of critical dimension are to be printed simultaneously, a pitch dependent variation of printed CD is observed. This phenomenon is called “iso-dense bias”.
The iso-dense bias may be caused by diffraction effects, which result is radiation reaching the substrate at locations other than those intended. The separation between two adjacent pattern features, namely the pitch, affects the impact of stray radiation from one pattern feature on the printing of an adjacent pattern feature. In addition, the overall intensity of the radiation projected onto the substrate may affect the iso-dense bias. In addition, substrate processing steps external to the lithographic apparatus, such as resist coating, resist development and etching, for example, can introduce an iso-dense bias in the pattern features formed on the substrate. In such chemical processing steps, the iso-dense bias is caused by the different concentrations of the reactive species in the isolated and dense pattern regions.